Tacky pressure-sensitive adhesive ("PSA") compositions suitable for use in adhesive tapes, for example, must have a requisite four-fold balance of adhesion, cohesion, stretchiness, and elasticity, an open time tack (i.e., period of time during which the adhesive is tacky at room temperature) on the order of days and often months or years, and a glass transition temperature (Tg) of less than about 20.degree. C. PSA-coated tapes have been produced for at least 50 years, however, early PSA-coated tapes did not have very desirable characteristics. Early PSA tapes were merely expected to temporarily adhere to a surface upon which they were adhered. Adhesive failure, discoloration, and cohesive failure were tolerated. Today, PSAs are expected to possess this extremely delicate balance of properties known in the trade as the "four-fold" balance of adhesion, cohesion, stretchiness, and elasticity. Some PSA compositions also have desirable transparency and resistance to aging, even upon exposure to severe weathering conditions.
Pressure sensitive adhesives have historically been based on petroleum-derived polymers such as poly(ethylene), poly(propylene), ethylene-vinyl acetate copolymers, and styrene block copolymers, for example. These compositions are typically further tackified, plasticized, and reinforced with a variety of resins, oils, and waxes, which are derived from both petroleum and naturally occurring feedstocks such as wood, gum, terpenes, etc. For example, early PSA compositions consisted of natural crude rubber tackified by esterfied wood rosin. However, these PSAs had poor aging properties, e.g., poor oxidative stability. These compositions were improved by the introduction of synthetic acrylic ester polymers, which were inherently tacky and possessed the elasticity and compliance required for the four-fold balance of properties. As the need arose, improvements were made in the basic acrylic ester PSA to meet the needs in the marketplace. Transparency and resistance to oxidation inherent in acrylic ester PSAs made them outstanding candidates for the most demanding PSA tape applications.
Environmental factors are becoming increasingly important in products marketed to consumers such as PSA containing diaper tapes, packaging tapes, medical tapes, surgical drapes, and the like. Two very important environmental factors are the mode of production and the mode of disposal of such products. For example, during manufacture, it is important to use solvent-free processing. Additionally, with the trend toward environmentally degradable materials, a PSA that could be disposed in an environmentally sound manner (e.g., in a municipal solid waste compost site) would be an important feature. The classic composition of PSAs are generally resistant to degradation upon disposal in such an environment. In addition to resisting degradation themselves, classic PSA materials can inhibit the degradation of the substrates on which they are coated. Thus, what is needed are PSAs, i.e., adhesives having the four-fold balance of properties described above, composed of biodegradable polymers.
Although hot melt adhesives, and even "pressure sensitive hot melt adhesives" (which are to be adhesives having a finite open time and lacking the four-fold balance of PSAs) composed of biodegradable polymers have been reported, little has been done in the area of biodegradable pressure sensitive adhesives. See, for example, U.S. Pat. Nos. 5,169,889 (Kauffman et al.) and 5,252,646 (Iovine et al.), which describe hot melt adhesives, varying from pressure sensitive to nonpressure sensitive in character, containing either poly(lactide) homoor copolymers or a linear polyester of 3-hydroxybutyric (HB) and 3-hydroxyvaleric acids (HV). The copolyesters, P(HB-co-HV), are statistically random and of high crystallinity (&gt;60%) throughout a range of compositions varying from 0 to 47 mol-% HV. See, for example, R. A. Gross et al., Macromolecules, 22, 1106-1115 (1989).